1. Field of the Invention
The present invention relates to a liquid electrophotographic printer and, more particularly, to an electrostatic transfer type liquid electrophotographic printer adopting a photoreceptor web as a photoreceptor medium.
2. Description of the Related Art
Electrophotographic printers such as laser printers output a desired image by forming a latent electrostatic image on a photoreceptor medium such as a photoreceptor drum or electroreceptor web, and developing the latent electrostatic image with a predetermined color toner. Electrophotographic printers are classified into a dry type or liquid type according to the toner used. For the liquid type printer, which uses an ink containing liquid carrier and solid toner in a predetermined ratio, it is easy to implement a color image with excellent print quality, compared with the dry type printer which uses solid toner. Electrophotographic printers are classified into a press transfer type and electrostatic transfer type according to the toner image transfer manner. To the press transfer type, after drying a toner image, the dried toner image is hot pressed by a transfer roller such that the image is transferred to a printer paper. The electrostatic transfer type printer transfers a toner image to a print paper by electric force.
FIG. 1 shows an example of a conventional electrostatic transfer type liquid electrophotographic printer, which adopts photoreceptor drums 10a, 10b, 10c and 10d as photoreceptor media. As shown in FIG. 1, this printer has a plurality of image forming units 1a, 1b, 1c and 1d for developing and transferring a predetermined color image to a print paper P. For a color printer, the four image forming units 1a, 1b, 1c and 1d for a color image development and transfer are arranged in a line in the direction of transferring the print paper P such that toner images are sequentially developed into four colors, yellow (Y), magenta (M), cyan (C), and black (K) to form a multi-color image. Reference numeral 2 denotes a feed belt 2 for feeding the print paper P.
The image forming units 1a, 1b, 1c and 1d include photoreceptor drums 10a, 10b, 10c and 10d on the surface of which a latent electrostatic image is to be formed, main chargers 20a, 20b, 20c and 20d being installed adjacent to the corresponding photoreceptor drums 10a, 10b, 10c and 10d to charge the surfaces of the photoreceptor drums 10a, 10b, 10c, and 10d to a predetermined potential, and laser scanning units (LSUs) 30a, 30b, 30c and 30d which scan light beams onto the surfaces of the respective photoreceptor drums 10a, 10b, 10c and 10d to form a latent electrostatic image thereon. Development units 50a, 50b, 50c and 50d which develop the latent electrostatic images into toner images with a predetermined color ink are installed below the respective photoreceptor drums 10a, 10b, 10c and 10d. Transfer chargers 70a, 70b, 70c and 70d which transfer the developed toner images formed on the respective photoreceptor drums 10a, 10b, 10c and 10d to a print paper P by electric force are spaced a predetermined distance apart from the surface of the corresponding facing photoreceptor drums 10a, 10b, 10c and 10d. 
The structure of the development units 50a, 50b, 50c and 50d will be described with reference to the development unit 50a for yellow (Y) toner image (referred to as Y-development unit 50a). Referring to FIG. 2, a developer roller 51, a squeeze roller 52 and a setting roller 53 are installed in the Y-development unit 50a. An ink supply unit 57 for supplying an ink to the developer roller 51 is installed adjacent to the developer roller 51. Scrapers 54, 55 and 56 are attached to the lower portion of the developer roller 51, the squeeze roller 52 and the setting roller 53, respectively, to scrap off the ink adhering to the surface of the corresponding rollers.
Development of a Y-toner image by the Y-development unit 50a having the configuration above will be described in greater detail. First, as the surface of the photoreceptor drum 10a charged to a predetermined potential by a main discharger 20a is irradiated by a light beam from the LSU 30a, a latent electrostatic image corresponding to the yellow color is formed. The developer roller 51 of the Y-development unit 50a rotates counterclockwise while being separated by a predetermined distance from the photoreceptor drum 10a. As an ink is supplied to the rotating developer roller 51 from the ink supply unit 57, the ink is carried to the gap between the photoreceptor drum 10a and the developer roller 51 by the rotation of the developer roller 51. The toner particles of the ink adhere to the latent electrostatic image formed on the photoreceptor drum 10a, so that a toner image is formed. At this time, the surface of the developer roller 51 is charged to a predetermined development potential such that the toner selectively adheres to only the latent electrostatic image, not to a non-image region.
The squeeze roller 52 removes excess liquid carrier from the photoreceptor drum 10a while being separated by a predetermined distance from the photoreceptor drum 10a and rotating clockwise.
The setting roller 53 rotates counterclockwise while being separated by a predetermined distance from the photoreceptor drum 10a, and creates an electric field between the photoreceptor drum 10a and the setting roller 53 with application of a predetermined voltage. The binding force between toner particles becomes strengthened by the electric field produced between the setting roller 53 and the photoreceptor drum 10a. Adhesiveness of the toner image to the photoreceptor drum 10a also increases. As a result, although an excessive amount of liquid carrier remains on the surface of the photoreceptor drum 10a for a subsequent electrostatic transfer, the shape and location of the toner image can be kept intact.
Once the toner image is set by the setting roller 53, the toner image is transferred to a print paper P by the electric field produced by the transfer charger 70a to which a potential is applied such that the transfer charger 70a is charged to the opposite polarity to the toner.
After a Y-toner image is transferred to the print paper P by the Y-image forming unit 1a, a magenta (M)-toner image is developed and transferred to the print paper P by the M-image forming unit 1b. 
As previously described, four toner images in Y, M, C and K are sequentially transferred to a predetermined area on the print paper P feed by a feed belt 2 in accordance with the print paper feed rate, so that a color image is printed on the print paper P. Because a large amount of liquid carrier remains on the resulting color image, a drying process is performed by a drying unit (not shown).
The conventional electrostatic transfer type liquid electrophotographic printer having the configuration described above has the following problems. First, since the conventional printer uses four photoreceptor drums as photoreceptor media, each for a particular color toner image, the multi-color toner images on the four photoreceptor drums must be sequentially transferred to a moving print paper with a predetermined time gap. The respective color toner images are separately transferred, and thus it is difficult to accurately transfer each of the color toner images in a particular area on the print paper in accordance with the print paper feed rate. In other words, an accurate registration control on the development and transfer processes performed by each image forming unit is difficult.
Second, four toner image transfer processes are carried out on a print paper feed by a feed belt, so that the print paper contacts the liquid carrier adhering to the surface of the photoreceptor drums four times. As a result, unnecessary consumption of the liquid carrier increases and the wetness of the print paper also increases.
Third, because the squeeze roller removes liquid carrier in a non-contact manner with respect to the photoreceptor drums, the amount of the liquid carrier remaining on the surface of the photoreceptor drums is nonuniform for all the image forming units. As a result, toner image transfer efficiency differs from color to color.
To address the above limitations, it is an object of the present invention to provide an electrostatic transfer type liquid electrophotographic printer which uses a photoreceptor web circulating around a continuous path as a photoreceptor medium.
To achieve the objective of the present invention, there is provided an electrostatic transfer type liquid electrophotographic printer comprising: a photoreceptor web circulating around a continuous path; a main charger for charging the surface of the photoreceptor web to a predetermined potential; a plurality of laser scanning units (LSUs) for sequentially forming a plurality of latent electrostatic images by scanning a light beam onto the charged surface of the photoreceptor web; a plurality of developer units arranged in series in the circulation direction of the photoreceptor web, for sequentially developing the plurality of latent electrostatic images into multi-color toner images with inks containing a liquid carrier and charged toner, thereby forming overlapping multi-color toner images on the photoreceptor web; a concentration control unit for controlling the concentration of the multi-color toner images to be suitable for electrostatic transfer by adjusting the amount of the liquid carrier applied to the overlapping toner images formed on the photoreceptor web; and an electrostatic transfer unit for forming an electric field between the photoreceptor web and the same and transferring the overlapping toner images formed on the photoreceptor web to a print paper by electric force.
In one embodiment, the concentration control unit may be installed in the last development unit of the plurality of the development units. It is preferable that the concentration control unit may be a concentration control belt rotating by being supported by at least two rollers while being separated by a predetermined distance from the photoreceptor web. Alternatively, the concentration control unit may be a concentration control roller having a diameter two times larger than the diameter of the developer roller, and rotating while being separated by a predetermined distance from the photoreceptor web.
In another embodiment, the concentration control unit may be spatially separated from the plurality of development units. In this case, the concentration control unit may include a carrier reservoir for storing a liquid carrier, and the concentration control belt or concentration control roller as described previously. The concentration control unit may further comprise a carrier supply nozzle for supplying the liquid carrier into the gap between the photoreceptor web and the concentration control belt. The concentration control belt and the concentration control roller allow the liquid carrier supplied into the gap between the photoreceptor web, and the concentration control belt and the concentration control roller to permeate into the toner images formed on the photoreceptor web.
The electrostatic transfer type electrophotographic printer according to the present invention may further comprise a setting roller for setting the shapes of the toner images formed on the photoreceptor web, wherein the surface of the setting roller is charged to a potential having the same polarity as the toner. It is preferable that the setting roller is installed while being separated from the photoreceptor web to the extent that the setting roller does not contact the liquid carrier layer on the photoreceptor web.
As the electrostatic transfer unit, an electrostatic transfer roller rotating in contact with the photoreceptor web, or a transfer charger installed facing to the photoreceptor web while being separated by a predetermined distance from the photoreceptor web may be used. A predetermined voltage, for example, of xe2x88x92900Vxe2x80x942 kV, having an opposite polarity to the toner, is applied to the electrostatic transfer roller and the transfer charger.
It is preferable that the electrostatic transfer type liquid electrophotographic printer further comprises a pre-conditioning unit for cleaning the surface of the photoreceptor web and forming a liquid carrier layer on the surface before development of the toner images.
According to the present invention, a color image can be obtained by sequentially forming multi-color toner images on the surface of the photoreceptor web, such that the toner images overlap each other. The multi-color toner images can be transferred to a print paper P by just one transfer process. Thus, registration in developing and transferring multi-toner images can be easily controlled. Also, wetness of the print paper and liquid carrier consumption decrease.